Electrostatic chuck having tapered electrodes

ABSTRACT

An electrostatic chuck is provided with an electrode split into two sections. The separate sections are joined through a tapered joint and bonded using epoxy. An insulator electrically separates the two sections and covers the top of the joined sections to form a clamping surface. The geometry of the electrode forms three annular regions, dividing the clamping surface into two equal areas distributed symmetrically. Moreover, the split-ring electrostatic chuck just described is fabricated by forming the two, separate electrodes; coating the separate electrodes with an insulator; joining the electrodes; machining the joint, top surface of the electrodes to form a single, co-planar, flat, smooth surface; and applying an insulator to that top surface.

BACKGROUND OF INVENTION

A. Field of Invention

The present invention relates, in general, to an electrostatic chuck forholding semiconductor wafers, especially silicon wafers, and to a methodfor fabricating such a device. More particularly, the present inventionrelates to a split-electrode electrostatic chuck in which electricalcontact to the wafer is not required and to a sequence of operationsused to manufacture that chuck.

B. Description of Related Art

In the manufacture of semiconductor devices, care must be taken whenprocessing the semiconductor wafer. Processing treatments may involvedirecting charged particles toward the wafer. For example, selectedareas of the wafer can have their conductivity type modified byimplanting ions. As another example, wafers formed of such materials asSi, SiO₂, Si₃ N₄, Al, W, No, Ti and the alloys of these metals and thelike may be dry etched using a plasma etch, a sputter etch, or areactive sputter etch.

Unfortunately, processing treatments involving the use of beams ofcharged particles generate thermal energy in the wafer. The problemscaused by such thermal energy include expansion and local distortion ofthe wafer and, in the case of a plasma etch, for example, melting of theresist used for the mask. In order to avoid these problems, the heatgenerated must be dissipated quickly. Additional processing problems arecreated because the wafers usually are not perfectly flat; they havewarps with lengths on the order of a few micrometers.

As is known in the art, improved processing of wafers can be achieved ifthe wafer is clamped substantially flat against a support base which istemperature controlled during treatment. By clamping the wafer, thenumber of points of contact between the wafer and the support isincreased; therefore, the thermal conductivity between the wafer andsupport is enhanced, warps are corrected, and the contact area isextended. The improved heat transfer, whether to or from the wafer,enables better control of the temperature of the wafer and, hence,better process control.

Accordingly, a variety of chucks have been proposed to clamp the waferduring processing. Such chucks include mechanical types, vacuum types,and electrostatic types. Electrostatic chucks are particularly useful inthe processes which produce semiconductor devices, however, because theycan be used under vacuum conditions, do not need mechanical structure tohold the wafer, and can apply a uniform clamping force.

Two general types of electrostatic chucks are known. In the first type,an electrode is positioned on a support base. An insulator is placedover the electrode and, in turn, the wafer is placed over the insulator.Voltage is applied between the electrode and the wafer, creating anelectrostatic force of attraction. Because the voltage is appliedbetween the wafer and the electrode, electrical contact with the waferis required. That requirement limits the material to be held, the wafer,to conductors, semiconductors, or materials at least covered with aconductive material on the surface. Thus, semiconductor wafers coveredwith an insulator such as an SiO₂ film cannot be clamped using thesingle electrode type of electrostatic chuck.

The second type of electrostatic chuck includes an electrode split intotwo or more areas, or separate electrodes, which can be held atdifferent potentials. The electrodes, typically planar, are positionedon a support base with a covering insulator and the wafer is placed onthe insulator. Voltage is applied between the electrodes, generating astrong electric field which produces an attractive force even if thewafer is coated with an insulator; the wafer is always at a differentpotential with respect to some part of the split electrodes, regardlessof what potential it may assume. Moreover, because the wafer is usuallyconductive, electrostatic capacities often exist between the wafer andthe electrodes.

When the electrode is split into two or more surfaces, problems arise.The force of attraction is very sensitive to small deviations, on theorder of a few microns, from the ideal, co-planar position of thesurfaces--as well as to surface roughness and flatness in general. It isdifficult to isolate the split electrode surfaces electrically yetmaintain those surfaces in a flat, co-planar relationship.

Further, in practice, the electrostatic chuck may be part of an rfdischarge apparatus. High frequency voltage (rf) is typically suppliedthrough the support and electrode on which the wafer rests. Thus, an rfcurrent flows through the support and electrode to ground or from an rfgenerator into the discharge through the support and electrode. That rfcurrent may generate different rf potential drops across the support andelectrode if the capacitive coupling between the two or more isolatedparts of the electrode is too low.

With the above discussion in mind, it is one object of the presentinvention to provide a split electrode electrostatic chuck having ageometrical design which improves the strength and uniformity of theclamping force of attraction between the chuck and wafer. A secondobject is to assure that the surface of the split electrode issubstantially a single, co-planar, flat, smooth surface. Also ofadvantage, and a further object, is a maximum thermal contact betweenthe wafer and support.

Still another object of the present invention is to provide a highcapacitance between the separate parts of the split electrode tominimize rf voltage drops. Another object is to assure the integrity ofthe insulator coating of the chuck during manufacture. A related objectis to protect the insulating coating of the chuck from the processingtreatment applied to the wafer.

SUMMARY OF THE INVENTION

To achieve these and other objects, and in view of its purposes, thepresent invention provides an electrostatic chuck having an electrodesplit into two sections. The separate sections are joined through atapered joint and bonded using epoxy. An insulator electricallyseparates the two sections and covers the top of the joined sections toform a clamping surface. The geometry of the electrode forms threeannular regions, dividing the clamping surface into two equal areasdistributed symmetrically.

The split-ring electrostatic chuck just described is fabricated byforming the two, separate electrodes; coating the separate electrodeswith an insulator; joining the electrodes; machining the joint, topsurface of the electrodes to form a single, co-planar, flat, smoothsurface; and applying an insulator to that top surface.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, but are notrestrictive, of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawings, in which:

FIG. 1 is a plan view of a split-electrode electrostatic chuckconstructed in accordance with the present invention;

FIG. 2 is a cross-sectional view of the split-electrode electrostaticchuck constructed in accordance with the present invention;

FIG. 3 is a plan view of a split-electrode electrostatic chuckconstructed in accordance with the present invention, illustratingcooling structure;

FIG. 4 is a cross-sectional view of the split-electrode electrostaticchuck constructed in accordance with the present invention taken alongline 4--4 in FIG. 3;

FIG. 5 is a plan view of the first electrode of a split-electrodeelectrostatic chuck constructed in accordance with the presentinvention;

FIG. 6 is a cross-sectional view of the first electrode of asplit-electrode electrostatic chuck constructed in accordance with thepresent invention taken along line 6--6 in FIG. 5;

FIG. 7 is a plan view of the second electrode of a split-electrodeelectrostatic chuck constructed in accordance with the presentinvention; and

FIG. 8 is a cross-sectional view of the second electrode of asplit-electrode electrostatic chuck constructed in accordance with thepresent invention taken along line 8--8 in FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a split-electrode electrostatic chuck 10 having two,separate electrodes. Second electrode 14 is shaped like a donut (seeFIGS. 7 and 8), having a substantially circular wall 16 defining acentral opening 18. As shown in FIGS. 5 and 6, first electrode 12 issubstantially disc-shaped and has a raised, central, hub portion 12awhich engages opening 18 in second electrode 14 and a raised, outer, rimportion 12b Which surrounds the outer edge 24 of second electrode 14.Thus, chuck 10 has a hub 12a and a rim 12b which are integrally (andelectrically) connected, but which are separated from the middle,donut-shaped electrode 14 of chuck 10.

As FIGS. 1 and 2 show, the clamping surface 26 of chuck 10 may be asubstantially circular disk, having a radius R, divided into three,annular regions. The boundaries on surface 26 separating those regionsare wall 16 and outer edge 24. Preferably, wall 16 is located at aradius of 1/4R and outer edge 24 is located at a radius of 3/4R. Thensurface 26 has two, separate, equal, electrode areas: the clampingsurface area of both electrode 12 and electrode 14 is (πR²)/2. Moreover,the separate clamping surface areas are symmetrically distributed, thusassuring a strong and uniform clamping force.

Electrodes 12 and 14 are coated with a suitable insulator 28, such asanodized aluminum, using a suitable method, such as anodization. Forexample, insulator 28 may be a hard coat applied using the Alumilite®(registration of the Aluminum Company of America) process that, at aminimum, meets the requirements of Military Specification No. MIL-A-8625Type III, Class 1 and produces a heavy, dense coating. Aluminum alloy6061-T6 is a preferable insulator 28.

The main requirement of the suitable insulator coating, whether it behard anodization, alumina, or any other insulator medium, and of thecoating process used, is that the integral coating be as non-porous, ordense, in crystalline structure as possible. The electrical or breakdownvoltage strength of insulator 28 should be at least 500 volts per mil.Insulator 28 should be applied to achieve a thickness of about 0.002inches.

The semiconductor wafer 1 (shown in FIG. 2 before it is held in positionon clamping surface 26) to be processed is held by the clamping surface26 of chuck 10 on top of insulator 28. The radius, R, of clampingsurface 26 is chosen to be smaller than the radius of the wafer by anamount sufficient to assure that the wafer will always cover surface26--within the bounds of possible wafer positions on chuck 10. Forexample, if the wafer has a radius of 62.5 millimeters, then the nominalwafer overhanq is 2.18 millimeters.

Moreover, a protective ring 30, which may be an anodized aluminum part,surrounds rim 12b of first electrode 12 of chuck 10. By assuring thatthe wafer will always cover surface 26 of chuck 10 and by providingprotective ring 30, the design of the present invention protectsinsulator 28 from the harmful effects of the processing treatmentapplied to the wafer. For example, in a plasma etch, the plasma isprevented from accessing insulator 28. The gap between the waferoverhang and ring 30 must be less than some critical dimension to insureprotection. In the embodiment of FIG. 1, that dimension is about 0.25millimeters.

In order to join first electrode 12 and second electrode 14 and formsplit-ring electrostatic chuck 10, a bond between the two electrodesmust be created. The integrity of that bond is critical; chuck 10 mustpresent substantially a single, co-planar, flat, smooth surface 26. Thepresent invention uses an epoxy 32 to form a bond between firstelectrode 12 and second electrode 14 and to prevent movement and providesealing.

First electrode 12 has a tapered depression 34 designed to accept band36 of second electrode 14. Depression 34 is tapered at an angle, α, ofabout ten degrees from normal (see FIG. 6). Band 36 is similarly taperedat an angle, β, of about ten degrees from normal (see FIG. 8). Bycorrespondingly tapering depression 34 and band 36, excess epoxy 32 canbe applied and squeezed out during bonding assembly. This leaves a verythin, void-free epoxy joint.

In many known designs, the electrode or electrodes are placed on asupport. In chuck 10 of the present invention, first electrode 12 isformed from a single, conducting block which also functions as a supportfor second electrode 14, insulator 28, and the wafer. An additional,separate support 44 is provided to support first electrode 12 and toclose the cooling channels 42 (see FIG. 4) formed in first electrode 12.As shown in FIG. 2, support 44 may be formed integrally with protectivering 30.

Processing treatments often generate thermal energy in the wafer, whichmust be dissipated. The design of chuck 10 outlined above assuresmaximum thermal contact between the wafer and electrodes 12 and 14,which act as a support for the wafer. By controlling the temperature ofelectrodes 12 and 14, the amount of heat exchanged between the wafer andelectrodes 12 and 14 can also be controlled. Such temperature control isprovided by two, separate systems.

First, as shown in FIGS. 3 and 4, chuck 10 has a network of gasdistribution grooves 38 throughout both first electrode 12 and secondelectrode 14. A gas supply tube 40 is provided vertically through bothfirst electrode 12 and second electrode 14 to connect grooves 38. Acooling gas, such as Helium, is passed through tube 40 into grooves 38to transfer heat from the wafer to first electrode 12 and secondelectrode 14.

Second, first electrode 12 of chuck 10 has a series of cooling channels42. A fluid such as water can be supplied to channels 42, furthercooling first electrode 12 and second electrode 14 and, in turn,dissipating the thermal energy generated in the wafer by the processingtreatment.

First electrode 12 and second electrode 14 are held, during operation,at different potentials. As shown in FIG. 2, a first electrical contact46 is provided to first electrode 12 and a second electrical contact 48is provided through clearance hole 49 to second electrode 14. Typically,first electrical contact 46 is a bolt which engages threaded aperture 47(see FIG. 4). Typically, second electrical contact 48 is a partiallythreaded metal rod which engages a threaded aperture in second electrode14. Voltage is applied between the electrodes, generating a strongelectric field which produces an attractive force clamping the wafer tosurface 26 of chuck 10.

Electrical connection may also be made to first electrode 12 byproviding a threaded hole in ring 30 for an electrical contact.Typically, the mounting bolts for chuck 10 act as the electricalconnection to first electrode 12. Second electrical contact 48 forsecond electrode 14 may be an electrically isolated through-hole (notshown) in ring 30 through which contact 48 passes. The diameter of thethrough-hole should exceed the diameter of the tapped hole in secondelectrode 14 to prevent surface leakage or breakdown. Note that noelectrical contact to the wafer is necessary.

The design of chuck 10 outlined above also assures high capacitancebetween first electrode 12 and second electrode 14. Such highcapacitance is achieved because the electrodes are in close proximity atboth their horizontal and tapered interfaces. It is desirable becausethe capacitance must be sufficiently great to render negligible andinduced rf voltage difference during operation. For the embodiment ofchuck 10 shown in FIG. 1, a capacitance in excess of 2,000 pf isacceptable for many applications. When the expected rf current flow isfive amperes (40.6 MHz), that capacitance creates an rf voltagedifference of 10 volts--which is tolerable. The required capacitance maybe more or less, however, depending upon the application and thefrequency.

Chuck 10 described above can be manufactured as follows. First electrode12 is made from a single, conducting block. Similarly, second electrode14 is made from a single, conducting block. The thickness of both firstelectrode 12 and second electrode 14 is oversized to allow latermachining, after joining, to form a single, co-planar surface. A gassupply tube 40, which will connect gas distribution grooves 38 whengrooves 38 are later formed, is machined vertically through firstelectrode 12 and second electrode 14. A series of cooling channels 42are also machined in first electrode 12.

The corners 50 on both first electrode 12 and second electrode 14 areradiused to assure coating integrity at the corners and to assure thatthe separation between the electrodes 12, 14 at their surface joints issufficiently large to prevent surface breakdown or leakage. The radii ofcorners 50 are typically 0.020 inches or larger. First electrode 12 andsecond electrode 14 are then coated with insulator 28, applied to athickness of about 0.002 inches, before joining.

An epoxy bond is formed between first electrode 12 and second electrode14. The tapered depression 34 of first electrode 12 is designed toaccept band 36, similarly tapered, of second electrode 14. Bycorrespondingly tapering depression 34 and band 36, excess epoxy 32 canbe applied and squeezed out during bonding assembly. The excess epoxythen, of course, is removed. This leaves a very thin, void-free epoxyjoint. The dimensions of depression 34 and band 36 are controlled toassure that mechanical contact between first electrode 12 and secondelectrode 14, through epoxy 32, is made at the bottom 52 of depression34 rather than at the sides 54 of depression 34. An epoxy-filled gap maybe maintained at sides 54, however, which is small--typically less thanthe thickness of insulator 28.

After joining first electrode 12 and second electrode 14, the topsurface 26 of assembled chuck 10 is machined free of the insulatorcoating. Grinding is not permitted, thus preventing oxide particles fromembedding in surface 26. The top surface 26 of the assembled chuck 10 ismachined to final dimension, smoothness, and flatness. A network of gasdistribution grooves 38 is then machined into the top surface 26.Grooves 38 are radiused to assure coating integrity of the finalinsulator 28 applied to surface 26. The smoothness and flatness ofsurface 26 are typically held to 16 microninches and to less than 0.0002inches, respectively. The outer border 56 of chuck 10 is provided with aradius, typically of 0.030 inches, to move the old insulator-newinsulator interface away from clamping surface 26.

Finally, a layer of insulator 28 is applied to the top, clamping surface26 of chuck 10. Such application may be by any suitable method,including anodization, as discussed above. The thickness of that finallayer is typically about 0.002 inches. Thus, the thickness of theinsulator 28 formed on the clamping surface 26 is between approximately0.001 and 0.003 inches.

The design of chuck 10 outlined above has been fabricated and operatedsuccessfully in an rf sputter-deposition device. The clamping forcecreated normally exceeds 10 Torr at an applied dc voltage of 650 voltsbetween first electrode 12 and second electrode 14. Such force allowsthe clamped wafer to be held and cooled properly. More particularly,when a plasma processing treatment is applied to the wafer, the forceallows the wafer to "float" to a dc potential determined by thatprocess.

Although the invention is illustrated and described herein as embodiedin a split-electrode electrostatic chuck having an electrode split intotwo sections, the separate sections joined through a tapered joint andbonded using epoxy, an insulator electrically separating the twosections and covering the top of the joined sections to form a clampingsurface in which the geometry of the electrode forms three annularregions to divide the clamping surface into two equal areas distributedsymmetrically, and in a method for fabricating such a chuck, theinvention is nevertheless not intended to be limited to the detailsshown. Rather, various modifications may be made in the details withinthe scope and range of equivalents of the claims and without departingfrom the spirit of the invention. For example, the chuck is describedherein as designed for circular semiconductor wafers, but could beapplied to other substrates.

What is claimed is:
 1. A split-ring electrostatic chuck for holding asemiconductor wafer, said chuck comprising:a substantially disc-shapedfirst electrode having a raised central hub portion and a raised outerrim portion forming a depression between said raised hub and said raisedrim, said depression having a taper; a second electrode having a donutshape with a substantially circular wall defining a central opening andan outer edge to form a band between said circular wall and said outeredge, wherein:(a) said hub of said first electrode engages said centralopening of said second electrode, (b) said rim of said first electrodesurrounds said outer edge of said second electrode, and (c) said band isadapted to engage said depression of said first electrode and is taperedto correspond to said taper of said depression; an insulatorelectrically separating said first electrode from said second electrodeand covering the tops of said first electrode and said second electrodeto form a clamping surface; a first electrical contact connected to saidfirst electrode; and a second electrical contact connected to saidsecond electrode, wherein a voltage is applied between said firstelectrode and said second electrode generating an attractive force forclamping said wafer to said clamping surface.
 2. A split-ringelectrostatic chuck as claimed in claim 1 wherein said taper of saiddepression formed between said raised hub and said raised rim of saidfirst electrode and said corresponding taper of said band between saidcircular wall and said outer edge of said second electrode are about tendegrees from normal.
 3. A split-ring electrostatic chuck as claimed inclaim 1 wherein said clamping surface defines two, separate, equalelectrode areas.
 4. A split-ring electrostatic chuck as claimed in claim3 wherein said equal electrode areas are symmetrically distributed.
 5. Asplit-ring electrostatic chuck as claimed in claim 1 wherein saidclamping surface is a single, co-planar, flat smooth surface.
 6. Asplit-ring electrostatic chuck as claimed in claim 1 wherein saidclamping surface of said electrostatic chuck has an outer boundary andsaid semiconductor wafer clamped by said electrostatic chuck on saidclamping surface has an outer boundary which extends beyond said outerboundary of said clamping surface.
 7. A split-ring electrostatic chuckas claimed in claim 1 further comprising a protective ring surroundingsaid rim of said first electrode of said electrostatic chuck.
 8. Asplit-ring electrostatic chuck as claimed in claim 7 wherein saidprotective ring is anodized aluminum.
 9. A split-ring electrostaticchuck as claimed in claim 1 wherein an epoxy bond is formed between saidfirst electrode and said second electrode.
 10. A split-ringelectrostatic chuck as claimed in claim 1 wherein no electrical contactis made to said wafer.
 11. A split-ring electrostatic chuck as claimedin claim 1 wherein a capacitance is generated between said firstelectrode and said second electrode, said capacitance being sufficientlygreat to render negligible the induced rf voltage difference.
 12. Asplit-ring electrostatic chuck as claimed in claim 1 wherein saidelectrostatic chuck has a network of gas distribution grooves throughsaid first electrode and said second electrode and a vertically disposedgas supply tube through said first electrode and said second electrodeconnecting said grooves.
 13. A split-ring electrostatic chuck as claimedin claim 12 wherein a cooling gas is passed through said gas supply tubeinto said network of gas distribution grooves to control the temperatureof the semiconductor wafer clamped by said electrostatic chuck on saidclamping surface.
 14. A split-ring electrostatic chuck as claimed inclaim 1 wherein said first electrode has a series of cooling channelsfor carrying a fluid.
 15. A split-ring electrostatic chuck as claimed inclaim 1 wherein said insulator has a thickness of between approximately0.001 and 0.003 inches.
 16. A split-ring electrostatic chuck as claimedin claim 15 wherein said insulator has a breakdown voltage strength ofat least 500 volts per mil.
 17. A split-ring electrostatic chuck asclaimed in claim 15 wherein said insulator is anodized aluminum.